Attenuation of Electromagnetic Radiation

Attenuation by scattering or pair-production processes is not treated at all in the present Volume. Attenuation by ionization of inner shells is treated in the Section on Ionization Processes and X-Ray Production , pp. 217/20. Compilations of individual and total photon cross sections (attenuation coefficients) for the whole range of photon energies were cited in, e.g., Tungsten Suppl. Vol. A2, 1987, pp. 69/77. 

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The attenuation of electromagnetic radiation as it passes through a sample is described quantitatively by two separate, but related terms transmittance and absorbance. Transmittance is defined as the ratio of the electromagnetic radiation s power exiting the sample, to that incident on the sample from the source, Pq, (Figure 10.20a). 

An alternative method for expressing the attenuation of electromagnetic radiation is absorbance. A, which is defined as... 

Looking at the phenomenon of optical absorption by the medium from the viewpoint of classical wave mechanics, we see that the attenuation of electromagnetic radiation can be attributed to the interaction of the oscillating electric vector with the medium. Any phenomenon involving periodic oscillations can be decomposed to real and imaginary components. Thus, the ordinary refractive index n is the real part of the index of refraction n, which can be written as... 

Fig. 13. Attenuation of electromagnetic radiation in the atmosphere. The solid curve indicates the altitude where the intensity of the external radiation is reduced to 1/2 of its original value at a given wavelength (after Giacconi et al., 1968). Physical phenomena and observational techniques are indicated for the different wavelength regions. All observed interstellar radio frequency lines lie within the atmospheric windows...

In absorption spectroscopy a beam of electromagnetic radiation passes through a sample. Much of the radiation is transmitted without a loss in intensity. At selected frequencies, however, the radiation s intensity is attenuated. This process of attenuation is called absorption. Two general requirements must be met if an analyte is to absorb electromagnetic radiation. The first requirement is that there must be a mechanism by which the radiation s electric field or magnetic field interacts with the analyte. For ultraviolet and visible radiation, this interaction involves the electronic energy of valence electrons. A chemical bond s vibrational energy is altered by the absorbance of infrared radiation. A more detailed treatment of this interaction, and its importance in deter-... 

The interaction of ionizing electromagnetic radiation with matter is different from the processes previously mentioned, and the concept of ranges and specific ionization loss cannot be applied. Only the three most important absorption processes are considered the photoelectric effect, the Compton effect, and the pair-production effect. The corpuscular description of electromagnetic radiation is the most appropriate for these effects, as one photon in a well-collimated beam of photons disappears at each interaction. The attenuation of the photon beam can be described by a simple exponential law... 

Every molecular species is capable of absorbing its own characteristic frequencies of electromagnetic radiation, as described in Figure 24-5. This process transfers energy to the molecule and results in a decrease in the intensity of the incident electromagnetic radiation. Absorption of the radiation thus attenuates the beam in accordance with the absorption law described later. 

Absorption A process in which a substance is incorporated or assimilated within another also, a process in which a beam of electromagnetic radiation is attenuated during passage through a medium. 

Turbidimetry relies on the attenuation of a radiation beam as a result of the scattering of electromagnetic radiation by suspended material formed during sample processing. No excited state is involved. Analogously to spectrophotometry, measurement of the power of the transmitted radiation allows the turbidimetric determination of the analyte. 

Exponential attenuation in the case of electromagnetic radiation, a = alpha P = beta X = X-ray... 

IR spectroscopy is one of the most powerful spectroscopic tools available for the analysis of polymer systems (a.l). IR spectroscopy is molecularly specific with high sensitivity. It is based on the absorption or attenuation by matter of electromagnetic radiation of a specified motion of chemical bonds. Through quantum physics, nature defines the absorption modes, their locations in the frequency spectrum and the amount of energy absorbed by each molecule. The absorbance at a characteristic frequency is a measure of the concentration of the chemical species being probed in the sample. 

Attenuation of radiation as it passes through the sample leads to a transmittance of less than 1. As described, equation 10.1 does not distinguish between the different ways in which the attenuation of radiation occurs. Besides absorption by the analyte, several additional phenomena contribute to the net attenuation of radiation, including reflection and absorption by the sample container, absorption by components of the sample matrix other than the analyte, and the scattering of radiation. To compensate for this loss of the electromagnetic radiation s power, we use a method blank (Figure 10.20b). The radiation s power exiting from the method blank is taken to be Pq. 

If a substance is irradiated with electromagnetic radiation, the energy of the incident photons may be transferred to the atoms or molecules raising them from the ground state to an excited state. This process, known as absorption, is accompanied by attenuation of the incident radiation at a particular frequency and can occur only when the energy difference between the two levels is exactly matched by the energy of the photons. The frequency of the radiation is given by... 

Figure 1.1 depicts the hard and soft fundamentals of process analysis. First, the process instruments range from simple to sophisticated measurement technologies. The majority of process instruments are based upon electromagnetic radiation attenuation as evident by the techniques described herein. Further discussion on real-time instramentation is provided in Section 1.3. 

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